This invention relates to an apparatus for controlling the revolutions (hereinafter referred to as RPM) of an internal combustion engine which serves to feedback-control a no-load RPM of the internal combustion engine to a predetermined RPM.
Heretofore, such a no-load RPM of an internal combustion engine has been subjected to a constant-value control to a predetermined RPM. The purposes of this RPM control are to set the no-load RPM small so as to suppress the fuel economy in a no-load mode as far as possible, and to suppress the fluctuation of the RPM ascribable to disturbance, so that a rapid controllability of high precision is required.
Factors for the fluctuations of the RPM are broadly classified into a primary factor attributed to the fluctuation of a no-load loss of the engine itself and/or to the fluctuation of the thermal efficiency of the engine, and a secondary factor attributed to the fluctuation of an adjustment gain inherent in a suction adjustment means employed for adjusting the RPM fluctuation caused by the primary factor and/or to the fluctuation of the density of the atmospheric air forming an air suction source.
As disclosed in the Official Gazette of Japanese Patent Application Laid-Open No. 59-162340, accordingly, there has been proposed a method of controlling the RPM of the engine to a target value by generating a target suction quantity or suction pipe pressure in accordance with an adjustment signal based on the deviation between the target value and actual value of the RPM and of controlling the suction adjustment means in accordance with an adjustment signal based on the deviation between the generated target and an actual suction quantity or suction pipe pressure.
According to this method, the adjustment signal (RPM adjustment signal) based on the deviation between the target value and actual value of the RPM attends to the primary factor of the RPM fluctuation, while the adjustment signal (suction adjustment signal) based on the deviation between the target value and actual value of the suction quantity or the suction pipe pressure separately attends to the secondary factor. It is therefore clear that the RPM fluctuations can be adjusted with higher precision and at higher speed than in the case of a feedback control based on only the RPM.
The above construction, however, has a major disadvantage to be stated below: When a failure has taken place upon either the detection of the RPM or the detection of the suction quantity, the corresponding adjustment signal responds limitlessly to cause the engine to runaway or stop abnormally and to fall into an unfavorable situation.